Apparatus and method for electric hot water heater primary frequency control

ABSTRACT

An energy storing apparatus is provided which comprises: a first region having a thermostat which includes: a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller; and a second region thermally coupled to the first region. An apparatus is provided which comprises: an interface to be coupled to an electric grid; a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller, wherein the power-converter is to couple via passive devices to a heating element, wherein the power-converter is to apply power to the heating element according to a temperature sensed by the temperature sensor and a frequency of the electric grid.

CLAIM OF PRIORITY

This application claims benefit of priority of U.S. ProvisionalApplication No. 62/239,765 filed 9 Oct. 2015, which is incorporated byreference in its entirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with support from the United States Governmentunder Grant No. 51205A (ECCS-0846533) awarded by the National ScienceFoundation (NSF). The Government has certain rights in the invention.

BACKGROUND

One limitation to large-scale integration of wind, solar, and marineenergy is their inherent variability. Presently, variability of the loador generation on the electrical grid is managed through fast acting(i.e., spinning) reserve generation. For large-scale integration ofvariable renewable power sources, the variability can exceed systemlimits. Energy storage is one means of managing the additionalvariability, but at present large dedicated energy storage systems aresignificantly more expensive than generation. A much more cost effectivesolution is to utilize demand response (i.e., controlling the electricalload to increase or decrease in response to the instantaneousavailability of renewable generation). One demand response resource thatstands out in its scope, flexibility, and simplicity is an electricwater heater.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the disclosure will be understood more fully from thedetailed description given below and from the accompanying drawings ofvarious embodiments of the disclosure, which, however, should not betaken to limit the disclosure to the specific embodiments, but are forexplanation and understanding only.

FIG. 1 illustrates a plot showing a transition drop in frequency for aWestern Grid Interconnect (WECC) model.

FIG. 2 illustrates a reduced-order model of WECC which includes systeminertia, load frequency response, and primary response of three classesof generators: slow, medium, and fast, according to some embodiments.

FIG. 3 illustrates a plot showing a comparison of the base casefrequency response and the reduced-order WECC model, according to someembodiments.

FIG. 4 illustrates a plot showing response of load for the three classesof generators to a simulated loss of generation.

FIG. 5 illustrates a water heater with upper and lower heating elements,according to some embodiments.

FIG. 6 illustrates a plot showing available aggregate water heater powercapacity as a function of aggregate water heater State of Charge (SOC).

FIG. 7 illustrates a simplified WECC model with aggregate water heatermodel, in accordance with some embodiments.

FIG. 8 illustrates a plot showing a frequency response of the WECC modelto a Palo Verde outage under three difference scenarios.

FIG. 9 illustrates a plot showing water heater power in response tochange in frequency due to loss of generation for two cases.

FIG. 10A illustrates a water heater with power electronics and control,according to some embodiments of the disclosure.

FIG. 10B illustrates an apparatus for the lower thermostat, according tosome embodiments of the disclosure.

FIGS. 11A-B illustrate a plot showing power and frequency simulationresults of 1000 water heaters over a 24-hour period.

FIG. 12 illustrates a water heater and operation of its thermostats,according to some embodiments of the disclosure.

FIG. 13 illustrates a circuit level architecture of the powerelectronics and control for the lower thermostat, according to someembodiments of the disclosure.

FIG. 14 illustrates an architecture of the lower thermostat, accordingto some embodiments of the disclosure.

FIG. 15A-B illustrate flowcharts of a method operation of the waterheater, according to some embodiments of the disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A powerful and flexible feature of residential hot water heaters is theability to heat both the upper part of the water tank—where the hotwater is withdrawn—and the lower part of the tank—where fresh cold wateris drawn in—separately with upper and lower heating elements. Someembodiments take advantage of this feature to provide a significantamount of energy storage in the lower part of the water tank, whilemaintaining the desired hot water temperature for the resident.

There are an estimated several million electric hot waters in thePacific Northwest of the United States of America. Each residential hotwater heater is typically rated, for example at 4.5 kW (kilo-Watt). Inthis example, if a 15 degree Celsius to 50 degrees Celsius temperaturerange is allowed in the bottom two-thirds of a typical residential waterheater tank, each water heater represents over 5 kWh (kilo-Watt hour) ofenergy storage. The total water heater potential for the PacificNorthwest is then in the range of 10 GW (Giga-Watt) of power capacityand 10 GWh (Giga-Watt hour) of energy storage.

In some embodiments, the energy storage potential of hot water heatersis realized by manipulating the temperature of the water in the tank. Anexample method is to heat the water to a high temperature when there isa large amount of renewable power available on the electric grid. Thisincreases the electric grid load to match the available renewable powergeneration. At a later time, when there is less renewable powergeneration available (e.g., the wind speed decreases and wind powerdecreases, or when sun sets and solar power decreases), the watertemperature is allowed to coast down to a lower temperature, thusallowing more water heaters to be off during this time, thus allowingthe electric grid load to better match the reduced renewable generation.

Stable operation of the electrical grid requires that the generatedelectrical power is equal to the load power. Deviations in this balance(in which the generation of electrical power may be greater than theload, or the load is greater than the generated electrical power) causethe rotating electrical machinery to increase or decrease in speed,which in turn increases or decreases the electric grid frequency. Largedeviations in the electric grid frequency can result in failure of theelectric grid connected equipment, or collapse of the entire electricgrid. Uncertainty in the generation of electric power or load imbalance,which can be caused by sudden loss of generation of electric power(e.g., from a failure of a generator), loss of transmission (e.g., likea tree failing on a power line), or changing wind conditions (e.g., whenwind speed changes causing wind power unit to stop generating power),can cause the electric grid frequency to deviate.

Special generation called “reserve” generation is kept alone toaccommodate for these changing conditions. Reserve generation tends tobe more expensive and less efficient than base load generation. Electrichot waters can provide some reserve generation functionality at a smallfraction of the cost of the building new electric generation facilities,in accordance with some embodiments.

The overall aggregate electrical load connected to the electric grid hasa frequency characteristic. The aggregate electrical load increasesslightly with an increase in frequency, and decreases slightly with adecrease in frequency. This is largely due to the electrical motorcomponent of the aggregate load, as these loads spin slightly fasterwith a higher grid frequency, and vice versa.

In some embodiments, an apparatus is provided that generates a frequencydependence in electric hot water heaters, which as non-rotating load donot have any frequency characteristics. As such, the apparatus allowsthe electric hot water heaters to participate in frequency regulation ofthe electric grid. The circuit(s) of various embodiments are responsiveto both temperature and electric grid frequency, lending some stabilityand inertia to the electric grid.

The apparatus of some embodiments retrofits existing water heaters, andbecause the apparatus operates on the electric grid frequency, noadditional communication signals (in or out) are needed. As such, insome embodiments, electric hot water heaters participate in frequencyregulation without the Internet or dedicated communicationinfrastructure.

Some embodiments provide mechanisms to allow the temperature of thebottom portion of the water heater tank to have a greater variation thanis currently allowed. In some embodiments, if the variation intemperature in the lower portion of the tank is kept within theappropriate bounds, it has little impact on the temperature in the topportion of the tank where water is withdrawn for the consumer. This isdue to stratification of water temperature layers in the tank, and thefact that the upper heating element has priority.

In some embodiments, the lower thermostat of an existing off-the-shelfresidential hot water heater is replaced with a low-cost embeddedcontroller and power converter. In some embodiments, the low-costembedded controller and power converter includes a temperature sensor.In some embodiments, the low-cost embedded controller and powerconverter is operable to support the operation of the lower part of thetank such that the electrical load of the water heater depends on thefrequency of the electrical grid. In some embodiments, the low-costembedded controller and power converter is a replacement of the standardthermostat in the lower part of the tank.

In some embodiments, the controller detects temperature of the lowertank, and also the frequency of the electrical grid (which is availableon the relayed power connection from the upper thermostat, for example).In some embodiments, when the electric grid frequency is above 60 Hz(Hertz), the power converter increases the power to the lower element.In some embodiments, when the electric grid frequency is below 60 Hz,the power converter decreases the power to the lower element.

There are many technical effects of the methods and apparatus/circuit(s)of the various embodiments. In some embodiments, the apparatus retrofitsexisting thermostat technology, effectively leveraging massive energystorage potential at a very low cost (e.g., effectively 5 kWh of energystorage capability is added for a cost of tens of dollars). In someembodiments, the apparatus can be easily installed in typical electrichot water heaters. In some embodiments, the apparatus allows the waterheater energy storage to participate in the stability of the electricgrid using merely the detection of electric grid frequency. In someembodiments, no additional communication overhead and control isnecessary (e.g., no Internet connections, no dedicated communicationnetworks and protocols are needed). The apparatus of the variousembodiments may not modify the proven and fundamental base controlscheme for hot water heaters. For example, the top or upper thermostatstill operates as before, and still has priority in maintaining desiredwater temperature for the consumer.

Various embodiments add frequency response to a large portion of theelectrical load that currently does not exhibit dependence on frequencyof the electrical grid. Load frequency response stabilizes the grid. Insome embodiments, the apparatus can be easily augmented to emulateinertia. Inertia stabilizes the grid.

In some cases, if balancing area operators allow some variation offrequency as a natural signal of renewable generation and load balance,the advanced hot water heaters of the various embodiments mayautomatically operate as energy storage devices to help integrate morerenewable power.

Here, the calibration of a reduced-order model of the Western GridInterconnect (WECC), and the usage of that model to investigate theimpact of large-scale integration of domestic hot water heaters forenergy storage and frequency response are described. The simulationssuggest that even a very modest penetration of water heaters (approx.500 MW (Mega-Watt) power capacity and 555 MWh (Mega-Watt Hour) energycapacity), can have a significant impact on reducing the frequencydeviation nadir and the settling frequency deviation in response to alarge power generation outage. With hot water heaters responding at thestandard of generation rated capacity per 3 Hz deviation, there is asmall but measurable reduction in the 60 Hz deviation nadir, from 59.666Hz to 59.673 Hz, for example.

If the water heaters are set to respond much more aggressively, e.g.,full rated capacity per 0.33 Hz deviation, the water heaters cancontribute their full 250 MW capacity (assuming 50% SOC—state of charge)and the frequency nadir improves from 59.666 Hz to 59.697 Hz, which is a10% improvement in the deviation from 60 Hz. Domestic hot heatersrepresent a very large, fast, and flexible resource that should be ableto be utilized with a much lower implementation cost than new energystorage.

Domestic water heaters represent a huge potential form of energystorage, both in terms of energy capacity and power capacity. In thisdisclosure, in some embodiments, merely the time scale that coversprimary response is considered. Generally, electric grid reservegeneration can be classified into four timescales described here.

First, inertial response is a timescale which is not dispatched, butinstead is the manifestation of the physical property that rotating massdischarges energy when decelerated, and charges energy when accelerated.

Second, primary control is a timescale which is the response ofgenerators under the control of governors to increase generation whenspeed (i.e., frequency) decreases and to decrease generation when speedincreases. This is a closed loop local-level control and the typicalresponse time is on the order of seconds. Primary response typicallyarrests, but does not correct, deviations in frequency. It is the firstactive layer of defense for grid stability.

Third, secondary control is a timescale in which additional generationset point operation is adjusted to correct for persistent frequencydeviation (i.e., drive the frequency back towards 60 Hz). Secondarycontrol timescales are typically minutes.

Fourth, tertiary control is a timescale which is loosely defined asadditional dispatcher corrections and scheduling, which typically covertimescales from minutes to hours.

Some embodiments here describe the development of a reduced-order modelof the WECC interconnection and the usage of the reduced-order model toestimate the benefit of domestic water heater participation in frequencyresponse (e.g., primary control) to assist in electric grid stabilityand recovery to large generator outages.

In some embodiments, a simple high-level model of the WECC is developedthat can recreate the WECC frequency response to a large generatoroutage with proper modeling of system inertia, load frequency response,and primary frequency response of generation.

In some embodiments, the model of WECC is then used to estimate thebenefit that large-scale water heater energy storage implementation canprovide to transient response and system stability.

The embodiments of the disclosure will be understood more fully from thedetailed description given below and from the accompanying drawings ofthe various embodiments of the disclosure, which, however, should not betaken to limit the disclosure to the specific embodiments, but are forexplanation and understanding only.

In the following description, numerous details are discussed to providea more thorough explanation of embodiments of the present disclosure. Itwill be apparent, however, to one skilled in the art, that embodimentsof the present disclosure may be practiced without these specificdetails. In other instances, well-known structures and devices are shownin block diagram form, rather than in detail, in order to avoidobscuring embodiments of the present disclosure.

Note that in the corresponding drawings of the embodiments, signals arerepresented with lines. Some lines may be thicker, to indicate moreconstituent signal paths, and/or have arrows at one or more ends, toindicate primary information flow direction. Such indications are notintended to be limiting. Rather, the lines are used in connection withone or more exemplary embodiments to facilitate easier understanding ofa circuit or a logical unit. Any represented signal, as dictated bydesign needs or preferences, may actually comprise one or more signalsthat may travel in either direction and may be implemented with anysuitable type of signal scheme.

Throughout the specification, and in the claims, the term “connected”means a direct connection, such as electrical, mechanical, or magneticconnection between the things that are connected, without anyintermediary devices. The term “coupled” means a direct or indirectconnection, such as a direct electrical, mechanical, or magneticconnection between the things that are connected or an indirectconnection, through one or more passive or active intermediary devices.

The term “circuit” or “module” may refer to one or more passive and/oractive components that are arranged to cooperate with one another toprovide a desired function. The term “signal” may refer to at least onecurrent signal, voltage signal, magnetic signal, or data/clock signal.The meaning of “a,” “an,” and “the” include plural references. Themeaning of “in” includes “in” and “on.” The terms “substantially,”“close,” “approximately,” “near,” and “about,” generally refer to beingwithin +/− 10% of a target value.

Unless otherwise specified the use of the ordinal adjectives “first,”“second,” and “third,” etc., to describe a common object, merelyindicate that different instances of like objects are being referred to,and are not intended to imply that the objects so described must be in agiven sequence, either temporally, spatially, in ranking or in any othermanner.

For the purposes of the present disclosure, phrases “A and/or B” and “Aor B” mean (A), (B), or (A and B). For the purposes of the presentdisclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B),(A and C), (B and C), or (A, B and C). The term “right,” “front,”“back,” “top” “bottom,” “over,” “under,” and the like in the descriptionand in the claims, if any, are used for descriptive purposes and notnecessarily for describing permanent relative positions.

For purposes of the embodiments, the transistors in various circuits andlogic blocks described here are metal oxide semiconductor (MOS)transistors or their derivatives, where the MOS transistors includedrain, source, gate, and bulk terminals. The transistors and/or the MOStransistor derivatives also include Tri-Gate and FinFET transistors,Gate All Around Cylindrical Transistors, Tunneling FET (TFET), SquareWire, or Rectangular Ribbon Transistors, ferroelectric FET (FeFETs), orother devices implementing transistor functionality like carbonnanotubes or spintronic devices. However, the embodiments are notlimited to these transistor types. Other types of transistors that canperform the various functions described here can also be used.

FIG. 1 illustrates plot 100 showing a transition drop in frequency for aWestern Grid Interconnect (WECC) model. Here, x-axis is time (inseconds) and y-axis is frequency (in Hertz (Hz)). For purposes ofdescribing the various embodiments, data for plot 100 comes primarilyfrom the 2011 California Independent System Operator frequency responsestudy, specifically the winter low-load high-wind case. The waveform 101is a base case which is for a total WECC load of 91,300 MW, with windproviding 14% of the electric power generation. Here, a loss of two PaloVerde units totaling 2,690 MW is simulated. This causes a transient dropin frequency around 10 seconds. The frequency nadir is 59.67 Hz at atime of 9.8 seconds after the loss of the Palo Verde units with asettling frequency of 59.78 Hz.

FIG. 2 illustrates reduced-order model 200 of WECC which includes systeminertia, load frequency response, and primary response of three classesof generators: slow, medium, and fast. These generators have a widerange of response times, from only seconds to well over a minute torespond to the Palo Verde unit loss.

Reduced-order model 200 comprises power model 201 to provide powerdisturbance 201 a (Pdisturbance); a subtraction unit 202, an attenuator203 to attenuate by ‘M’ (e.g., 1/M); integrator or frequency converter204 (e.g., 1/s) to provide per unit frequency 204 a (e.g., running timeintegral of the input); bias generators including: load bias generator205 (loadBias), fast bias generator 206 (genBiasFast), medium biasgenerator 207 (genBiasMed), and slow bias generator 208 (genBiasSlow);load response transfer function block 209 (load_resp=1/(Tload.s+1));fast bias generation response transfer function block 210(genf_resp=1/(Tgenf.s+1)); medium bias generation response transferfunction block 211 (genm_resp=1/(Tgenm.s+1)); slow bias generationresponse transfer function block 212 (gens_resp 1/(Tgens.s+1)); andsummer 213. [

The output of block 209 is load power 209 a (Pload) which is received bysubtraction unit 202. The output of block 210 is fast generation power210 a (Pgenfast) which is received by summer 213. The output of block211 is medium generation power 211 a (Pgenmed) which is received bysummer 213. The output of block 212 is slow generation power 212 a(Pgenslow) which is received by summer 213. The output 213 a of summer213 is received by subtraction unit 202.

In some embodiments, the model parameters the various blocks of FIG. 2are tuned by successive sweeps, until a frequency response fit of acertain threshold is found (e.g., 0.01 Hz RMSE (Root-Mean-Square Erroror Deviation)). Model parameters for some embodiments are given in TableI.

TABLE I MODEL PARAMETERS Symbol Name Value Units M Inertia 15 PUpwr-s/PU freq loadBias Load bias 1.027 PU pwr/PU freq genBiasFast Fastgen. bias 3.432 PU pwr/PU freq genBiasMed Medium gen. bias 2.112 PUpwr/PU freq genBiasSlow Slow gen. bias 1.056 PU pwr/PU freq Tload Loadresp. time const. 0 seconds Tgenf Fast gen. time const. 0.5 secondsTgenm Med. gen. time const. 19 seconds Tgens Slow gen. time const. 27seconds Pbase Sim. power base 92 GW

FIG. 3 illustrates plot 300 showing a comparison of the base casefrequency response and the reduced-order WECC model. Here, x-axis istime (in seconds) and y-axis is frequency (in Hertz (Hz)). Waveform 301is the same as waveform 101 (e.g., the base case frequency response)while waveform 302 is the transient response of the WECC model of FIG.2.

FIG. 4 illustrates plot 400 showing response of a load for the threeclasses of generators to a simulated loss of generation. Here, x-axis istime (in seconds) and y-axis is power (in Giga Watts (GW)). Plot 400illustrates response of the lost generation (waveform 401), response ofthe fast generator (waveform 402), response of the slow generator(waveform 403), response of the medium generator (waveform 404), totalresponse of the fast, slow and medium generators (waveform 405),response of the load (waveform 406), and response of the base case(waveform 407).

Here, the term “lost generation” generally refers to an amountgeneration that went away when the simulated Palo Verde generation unitsfailed. For example, if a 100 MW generator failed and tripped offline,which would be 100 MW of lost generation. In other words, it is theamount of generation that must be made up from other sources tostabilize the grid.

In this example, the fastest generator group (waveform 402) reaches itsmaximum response around 10 seconds, closely following the frequency ofthe system. The slow speed and medium speed generator groups (waveforms403 and 404, respectively) reach their maximum generation in 40 secondsand 60 seconds after the disturbance, respectively. The total generation(waveform 405) is the sum of these three groups and very closely matchesthe base case generation response (waveform 407).

FIG. 5 illustrates water heater 500 with upper and lower elements. Thewater heater 500 comprises tank 501, cold water inlet 502, hot wateroutlet 503, upper thermostat 504, upper heating element 505, lowerthermostat 506, and lower heating element 507. Electric water heatersare a significant energy storage resource. Standard residential electricwater heaters are 50 or 60 gallons capacity, with one heating element atthe bottom of the tank, and another approximately two-thirds up thetank.

The hot water outlet 503 is at the top of tank 501. Upper element (orupper heating element) 505 has priority and heats the upper one-third oftank 501 to the desired temperature, and possibly as high as 140 degreesFahrenheit. Lower element (or lower heating element) 507 can be activewhen upper element 505 is off (e.g., when the upper one-third of tank501 is at the desired temperature) and heats the lower two-thirds oftank 501 to the desired temperature. Generally, the water stays wellstratified, and the lower two-thirds of tank 501 can be assumed to holda temperature below that of the upper one-third of tank 501. Thisstructure allows for great opportunities for utilizing electric waterheaters as energy storage. In some embodiments, the temperature of thelower two-thirds of tank 501 can be manipulated in an intelligent waywithout greatly affecting the temperature of the water in the upper onethird of tank 501, which is the water withdrawn by the user.

Assuming a standard residential electric water heater of 50 gallonscapacity, an outlet temperature of 130 degrees Fahrenheit, and an inlettemperature of 60 degrees, the lower two-thirds of tank 501 may requireapproximately 5 kWh of energy to heat. Therefore, by controlling thelower two-thirds of the tank temperature to range from the inlettemperature as the lower bound, and the outlet temperature as the upperbound, a potential energy storage capacity of 5 kWh can be utilized.

Typically, each of upper and lower elements 505 and 507, respectively,are rated for 4.5 kW. Under the standard configuration, upper element505 has priority to heat the upper portion of tank 501. If upper element505 is off, lower element 507 is enabled, in accordance with someembodiments. Therefore, the two elements (505 and 507) operateexclusively, and the total water heater power capacity is 4.5 kW, forexample. When referring to water heaters generating (e.g., discharging),it may not mean they push energy on the electric grid, but insteaddecrease their load by the amount they are effectively generating, as ifsome local generation source has come online to decrease the net load onthe electric grid.

In some embodiments, the lower thermostat 506 is replaced with alow-cost embedded controller and power converter. In some embodiments,the low-cost embedded controller and power converter includes atemperature sensor. In some embodiments, the low-cost embeddedcontroller and power converter is operable to support the operation ofthe lower part of tank 501 such that the electrical load of the waterheater depends on the frequency of the electrical grid. In someembodiments, the low-cost embedded controller and power converter is areplacement of the standard thermostat in the lower part of the tank.

A single water heater could be modeled as a battery or capacitor. Insome embodiments, an aggregate of millions of water heaters are modeled.The aggregate system has a power capacity, and a state of charge (SOC).Contrary to a single battery or capacitor, the power capacity of theaggregate system is a function of the SOC.

For example, consider the SOC of the entire aggregate system to be 0.5(i.e., half full). In that case, some of the water heaters will be fullycharged, some partly charged, and some fully discharged, such that thetotal state of charge is 0.5. However, the water heaters that are fullycharged may not be able to contribute to total charging power capacity,and the water heaters that are fully discharged may not be able tocontribute to total discharging power capacity.

Consider the case of the total system SOC at 0.9. In that case, thereare many water heaters that are fully charged, a few that are partlycharged, and a very few that are fully discharged. Thus the totalcharging power capacity will be small, and the total dischargingcapacity will be large. Therefore, for the aggregate system of millionsof water heaters, it is expected for the charging and discharging powercapacity to be a function of the total aggregate SOC.

FIG. 6 illustrates plot 600 showing available aggregate water heaterpower capacity as a function of aggregate water heater SOC. Here, x-axisis SOC (e.g., 0, 0.5., 1), and y-axis is power rating P_(rated) (e.g.,power capacity of water heater (wh) P_(wh,totalcapacity) and-P_(wh,totalcapacity)). For purposes of simplicity, a linearrelationship of power capacity to SOC is assumed. For a given SOC, line601 is the maximum charging power available (e.g., positive powerP_(charge)), and line 602 is the maximum discharging power (e.g.,negative power P_(discharge)) available. When the aggregate SOC is zero,the lower tanks of all the water heaters are at their lowesttemperature, and all are available for charging, in accordance with someembodiments. When the aggregate SOC is one, the lower tanks of all thewater heaters are at their highest temperature, and all are availablefor discharging, in accordance with some embodiments. When the aggregateSOC is 0.5, it is approximated that half of the water heaters areavailable for charging, and half are available for discharging, inaccordance with some embodiments.

FIG. 7 illustrates a simplified WECC model 700 with aggregate waterheater model, in accordance with some embodiments. WECC model 700comprises reduced-order model 200, block 701 (which modelsPwhTotCap*60/c_Hz/Pbase), block 702 (which models PwhTotCap/Pbase),block 703 (which models PwhTotCap/Pbase), first summer block 704, block705 (which is a saturation block which caps the incoming signal ‘u’ tothe limits set by the inputs “Up” and “Lo”), block 706 (which modelsPwhTotCap/Pbase), block 707 (which models a zero), second summer block708, block 709 (which denotes running time integration 1/s), and block710 (which models Pbase/(JwhTotCap*3600) coupled together as shown.Block 705 models the aggregate water heater characteristics illustratedby FIG. 6, in accordance with some embodiments. Referring back to FIG.7, here, output 701 a of block 701 is Pwh_cmd. Output 704 a of firstsummer block 704 is PchargeCapacity. Output 708 a of block 708 isPdischargeCapacity. Output 705 a of block 705 is Pwh, and output ofblock 710 is SOC.

The water heater parameters (e.g., PwhTotCap, c_Hz, Pbase, PwhTotCap,Pbase, 1/s, JwhTotCap) are given in Table II. The blocks (e.g., 701-710)in the upper right of FIG. 7 model the water heater power limits, whichare linear functions of SOC illustrated in FIG. 6. The commanded waterheater power “Pwh_cmd” from block 701 is the desired water heater powerabove or below the nominal natural water heater load at the moment. Theamount of increase or decrease in the water heater load is dictated bythe control variable “c_Hz”, which specifies the amount of commandedwater heater power per deviation in frequency.

In some embodiments, the goal is to show the benefit to transientresponse that water heater loads can provide. In some embodiments, thewater heater power is to be controlled proportional to frequency, thusproviding primary frequency response. The frequency may then drop belowa threshold (e.g., 60 Hz) due to the simulated loss of the Palo Verdeunits (e.g., 2.69 GW=0.029 PU), some of the water heater capacity mayrespond by decreasing load, thus helping to stabilize the system.

In some embodiments, for a water heater control system, the lowerelement 507 of the water heater is replaced by an augmented heatingelement that includes a circuit to detect frequency of the electricalgrid. In some embodiments, as the frequency of the electrical griddecreases, the effective element resistance—and therefore the elementpower—can be effectively changed by pulse width modulating (PWM) theapplied voltage. If the home resident is in need of hot water, upperelement 505 will be on and lower element 507 is unpowered. In someembodiments, if the upper tank is standing by ready with the desiredwater temperature, lower element 507 will be available, and thus with asimple augmentation of the thermostat to respond to frequency, will besilently increasing or decreasing the temperature in the lower tankaccording to the frequency of the electrical grid.

By staying within this traditional water heater control framework, theupper tank and home resident comfort remains the priority. (It is noted,however, that deep discharging of the SOC—i.e., a low temperature of thelower tank—could overwhelm the ability of the upper element to keep theoutlet water at the desired temperature).

In some embodiments, this modification may require no additionalcommunication interface. For example, the modification would effectivelyutilize frequency to detect the state of the generation-load balance.Under this scheme, in some cases, the response of any one individualwater heater cannot be guaranteed. However, the aggregate water heaterbehavior of millions of water heaters is predictable, in accordance withsome embodiments.

It is estimated there are approximately 3 to 4 million residential waterheaters in the Pacific Northwest of the United States. Assuming eachwater heater to have a power capacity of 4.5 kW, there is a totalresidential water heater load capacity of approximately 15,000 MW. Forpurposes of simplicity, here a very conservative estimate of 3% is madeof that resource (e.g., 500 MW) to be available for demand response(e.g., primary frequency participation) for the entire WECC. Assumingeach water heater to have a power capacity of 4.5 kW, that is a total of111,000 water heaters, and further assuming 5 kWh per water heater,there is a total energy storage resource of 555 MWh.

Here, two scenarios are simulated: First, the aggregate water heaterload responding at the standard droop of 5%, and Second a moreaggressive scenario of the aggregate water heater load responding at adroop of 0.55%. For the 5% droop case—which is a standard setting forgeneration on primary frequency control—the generator governor is set tomodify the generator power set point at a rate of Prada per 3 Hz. Forthe 0.55% case, the generation set point is modified at a rate of Pradaper 0.33 Hz. Generally,

$\begin{matrix}{{P(f)} = {P_{setpoint} + {P_{rated}\frac{f}{c_{Hz}}}}} & (1)\end{matrix}$

For the water heater, control P_(setpoint) is simply the current naturalaggregate water heater load, which is treated as zero in the simulation.(Note that zero load in the simulation does not represent objectivelyzero Watts, but is instead the bias point of the simulation.) Theparameter c_(Hz) (or c_Hz) is 3 Hz for the 5% case, and 0.33 Hz for the0.55% case. A value of 0.33 Hz is chosen in this example as that is thefrequency deviation nadir of the benchmark case. Thus when cHz is 3Hz,approximately 0:33/3=11% of the total aggregate water heater capacitywill be utilized, whereas when cHz is 0.33 Hz, approximately 100% willbe utilized.

The aggregate water heater parameters are given in Table II.

TABLE II WATER HEATER PARAMETERS Symbol Name Value Units PwhTotCap Totalaggregate water heater 500 MW power capacity JwhTotCap Total aggregatewater heater 555 MWh energy capacity c_Hz Water heater freq. response{0.33, 3.00} Hz constant

FIG. 8 illustrates plot 800 showing a frequency response of the WECCmodel of FIG. 7 to a Palo Verde outage under three difference scenarios.It is pointed out that those elements of FIG. 8 having the samereference numbers (or names) as the elements of any other figure canoperate or function in any manner similar to that described, but are notlimited to such. Here, x-axis is time (in seconds) and y-axis isfrequency (in Hertz). Plot 800 shows the frequency response of the WECCto the Palo Verde outage (e.g., 2.69 GW of lost generation) under threescenarios. The first scenario is the base case of no water heater energystorage which is illustrated by waveform 801. The second scenario is thestandard case of all available water heaters responding for 3 Hz (e.g.,5%) deviation, which is illustrated by waveform 802. The third scenariois the aggressive case of all available water heaters responding for a0.33 Hz (e.g., 0.55%) deviation, which is illustrated by waveform 803.The frequency nadirs and settling times are summarized in Table III.

TABLE III COMPARISON OF FREQ. RESP. WITH WATER HEATERS PARTICIPATINGCase Freq nadir Settling freq Base 59.666 Hz 59.768 Hz c_Hz = 3 Hz59.673 Hz 59.771 Hz c_Hz = 0.33 Hz 59.697 Hz 59.789 Hz

The initial aggregate water heater SOC is 0.5, and therefore 250 MW ofthe 500 MW is available for charging, and 250 MW is available fordischarging. The results show that for the standard case of full waterheater response per 3 Hz of frequency deviation as shown by waveform802, only about ⅕ (e.g., 50 MW) of the available 250 MW is utilized.However, even with this modest response, there is a small but measurablebenefit in the frequency nadir and settling frequency. For the moreaggressive case of full water heater response per 0.33 Hz of frequencydeviation as shown by waveform 803, the full water heater capacity of250 MW is utilized. The benefit is much greater in this case, with thefrequency nadir increased from 59.666 Hz to 59.697 Hz over the base case(which is waveform 801), which is a 10% improvement in the deviationfrom 60 Hz. The SOC is not plotted, but in this example it iseffectively unchanged over the 60 second simulation time span as theenergy storage capacity of 555 MWh is so large.

FIG. 9 illustrates plot 900 showing water heater power in response tochange in frequency due to loss of generation for two cases. Here,x-axis is time (in seconds) and y-axis is Power (in MW). Plot 900 showswater heater power in response to the change in frequency due to theloss of 2.69 GW of generation for two cases. The first case isillustrated by waveform 901 which is the case for all available waterheater power responding per 3 Hz frequency deviation. The second case isillustrated by waveform 902 which is the case for all available waterheater power responding per 0.33 Hz deviation.

FIG. 10A illustrates a water heater 1000 (also referred to as an energystoring apparatus) with power electronics and control, according to someembodiments of the disclosure. It is pointed out that those elements ofFIG. 10A having the same reference numbers (or names) as the elements ofany other figure can operate or function in any manner similar to thatdescribed, but are not limited to such. FIG. 10A is described withreference to other figures including FIG. 5.

Standard electric hot water heater control relies on chained operationof the upper tank and lower tank relays. Here, the upper tank is alsoreferred to as the first region while the lower tank is referred to asthe second region, where the first region is thermally coupled to thesecond region. The upper tank thermostat/relay 1001/504 has priority, asthe hot water is drawn from the upper tank. If the upper tanktemperature is below its lower limit, the upper tank relay is closed(e.g., connected) and the upper tank heating element is energized, andthe lower element is off. If the upper tank temperature is above itsupper limit, the upper tank element is de-energized, and the lowerthermostat is active. If the lower thermostat is active, if the lowertank temperature is below its lower limit, the lower element isenergized. If the lower tank temperature is above its upper limit, thelower element is de-energized. In this standard mode of operation, theupper tank heating has priority, and merely one of the two 4.5 kWheating elements is active at a time. In some embodiments, the firstregion has set point which is set to a temperature lower than atemperature of the second region.

In some embodiments, the operation of upper thermostat/relay 1001/504 isunchanged. The upper thermostat/relay 1001/504 comprises thermostat 1001a coupled to 220-240 V input 1009 via wires 1005 and 1006. Wires 1007and 1008 are coupled to upper thermostat/relay 1001/504 and lowerthermostat/relay 1003/506. The upper thermostat/relay 504 is coupled tothe upper heater element 1002/505. The lower thermostat/relay 1003/506is coupled to the lower heater element 1004/507. The lowerthermostat/relay 506 comprises the thermostat 1003 a. In someembodiments, the lower thermostat/relay 1006/506 includes a “smart”component of a same physical form-factor as a traditional lowerthermostat.

FIG. 10B illustrates apparatus 1020/1003/506 for the lower thermostat,according to some embodiments of the disclosure. It is pointed out thatthose elements of FIG. 10B having the same reference numbers (or names)as the elements of any other figure can operate or function in anymanner similar to that described, but are not limited to such. In someembodiments apparatus 1020/1003 comprises micro-controller 1021 (orcontroller), a power-converter 1022 that controls the amount of powerapplied to lower element 507, and temperature sensor 1023 coupledtogether as shown. In some embodiments, micro-controller 1021 is one of:A Digital Signal Processor (DSP), an Application Specific IntegratedCircuit (ASCI), a general purpose Central Processing Unit (CPU), or alow power logic implementing a simple finite state machine to performthe various methods described here. In some embodiments, power-converter1022 is to control the amount of power applied to the first region. Insome embodiments, power-converter 1022 is a buck converter or any otherDC-DC converter. In some embodiments, power-converter 1022 is operableto apply power to the first region according to temperature sensed bytemperature sensor 1023 and a frequency of the electric grid.

In some embodiments, the power applied to lower element 507 is appliedbased on two considerations: temperature and grid frequency. In someembodiments, the two proportional loops (first loop and second loop)regulating temperature and grid-frequency are operated in parallel. Insome embodiments, the first proportional loop is to regulate atemperature sensed by temperature sensor 1023. In some embodiments, thesecond proportional loop regulates rate-of-change of grid frequency toemulate inertia of the electric grid.

For example, the second proportional loop adjusts a frequency of theelectric grid. In some embodiments, the first and second proportionalloops operate in parallel. In some embodiments, the lower tanktemperature set-point is set to a temperature slightly lower than theupper tank, to allow for range of operation above the set-point when thegrid frequency is low, without exceeding the tank maximum temperature.In some embodiments, when the electric grid frequency is high, it willtend to depress the lower tank temperature, until thefrequency-regulating loop reaches equilibrium with the temperatureregulating loop.

FIGS. 11A-B illustrates plots 1100 and 1120 showing power and frequencysimulation results of 1000 water heaters over a 24-hour period. For plot1100, the x-axis is time in hours while the y-axis is power in kW. Trace1101 is the aggregate load power of water heaters without the lowerthermostat replacement (e.g., using a traditional lower thermostat).Trace 1102 is the aggregate load power of the water heaters with lowerthermostat 506. For plot 1120, the x-axis is time in hours while they-axis is power in kW. Trace 1121 is the grid frequency. Severalfeatures are exhibited by these traces. For example, at approximately 6hours, there is a large grid frequency disturbance. This disturbancecauses a nearly instantaneous stabilizing response in the water heaterload. From hour 16 to hour 20 along the x-axis, the grid frequency isslightly below 60 Hz, which causes a corresponding decrease in theaggregate water load of the demand response enabled water heaters. Fromhour 22 to hour 24 along the x-axis, the grid frequency is above 60 Hz,and the demand response enabled water heater load is large. In hours16-20, while the frequency is slightly below 60 Hz, waveform 1102increases in that period, at least over the first two hours. It goesfrom about 500 kW at approximately 16 hours to approximately 750 kW byabout 18 hours. At 22 hours, frequency ticks up slightly, and then thedemand response decrease from about 600 kW to below 400 kW over thattime period.

FIG. 12 illustrates water heater 1200 and operation of its thermostats,according to some embodiments of the disclosure. It is pointed out thatthose elements of FIG. 12 having the same reference numbers (or names)as the elements of any other figure can operate or function in anymanner similar to that described, but are not limited to such. Waterheater 1200 is same as water heater 1000. Here, upper thermostat block504 comprises relay 1201 and thermostat 1203 coupled via wire 1202. Aninterface to the grid is also provided. Here, lower thermostat block 506comprises thermostat 1204 coupled to lower element 507 via wires 1206and 1207. An interface to the electric grid is also provided. Plot 1209shows hysteresis between On and Off times for lower thermostat 506. Plot1210 shows hysteresis between On (curve 1210 a) and Off (curve 1210 b)times for upper thermostat 504.

FIG. 13 illustrates circuit level architecture 1300 of the powerelectronics and control for lower thermostat 506, according to someembodiments of the disclosure. It is pointed out that those elements ofFIG. 13 having the same reference numbers (or names) as the elements ofany other figure can operate or function in any manner similar to thatdescribed, but are not limited to such

In some embodiments, architecture 1300 comprises diode bridge rectifier1301, driver 1302, and control block 1303. In some embodiments, controlblock 1303 switches between T* and T based on frequency ‘f’. The outputof control block 1303 is coupled to driver 1302 which provides controlsignals for controlling gate terminals of transistors in circuit 1301.In some embodiments, circuit block 1301 comprises inductor capacitor‘C’, first n-type transistor MN1, second n-type transistor MN2, anddiodes D1, D2, D3, and D4 coupled together as shown. Here, inductor L,capacitor C, and transistors MN1 and MN2 together form an analog portionof a buck converter or power converter. The diodes D1, D2, D3, and D4perform the function of a rectifier.

In some embodiments, inductor ‘L’ and capacitor ‘C’ are coupled to lowerheating element 507 via nodes 1206 and 1207. One end of inductor ‘L’ iscoupled to node n1 which is a common node coupling transistors MN1 andMN2. The gate terminal of MN1 is controlled by “g2” provided by driver1302. The gate terminal of MN2 is controlled by g1 (or V₁) provided bydriver 1302. Diodes D1 and D2 are coupled between supply Vdd and node1207. Diodes D3 and D4 are coupled between supply Vdd and node 1207. Thenode coupling diodes D1 and D2 is coupled to the electric grid. The nodecoupling diodes D3 and D4 is coupled to the electric grid. The transferfunction of the inductor and capacitor network is shown by plot 1304,where input is V₁ and output is V₂. The rectified utility voltage (e.g.,voltage across Vdd and node 1207) is shown by plot 1305. The transientresponse at the nodes coupled to the electric grid is given by 1306.

FIG. 14 illustrates architecture 1400 of the lower thermostat, accordingto some embodiments of the disclosure. It is pointed out that thoseelements of FIG. 14 having the same reference numbers (or names) as theelements of any other figure can operate or function in any mannersimilar to that described, but are not limited to such. Architecture1400 illustrates the apparatus that inputs to control block 1303. Insome embodiments, architecture 1400 includes step down converter (ortransformer) 1401, analog-to-digital converter (ADC) 1402, phase lockedloop (PLL) 1403, adder/subtractor 1404, gain stage 1405 (e.g., havinggain K₁), saturation block 1406 adder/subtractor 1407, gain stage 1408(e.g., having gain K₂), adder/subtractor 1409, and sampler 1411 coupledtogether as shown. Saturation block 1406 limit the range of the desiredtemperatures (T*) to a safe upper and lower limit, in accordance withsome embodiments. Saturation block 1406 may also limit “D*” to between 0and 1, in accordance with some embodiments.

In some embodiments, an antenna (not shown) is provided to send andreceive information associated with apparatus 1400. For example, variouscontrol parameters can be read, set or adjusted by wireless means usingthe antenna. In some embodiments, the antenna may comprise one or moredirectional or omnidirectional antennas, including monopole antennas,dipole antennas, loop antennas, patch antennas, microstrip antennas,coplanar wave antennas, or other types of antennas suitable fortransmission of Radio Frequency (RF) signals. In some multiple-inputmultiple-output (MIMO) embodiments, Antenna(s) 101 are separated to takeadvantage of spatial diversity.

ADCs are apparatuses that convert continuous physical quantities (e.g.,voltages) to digital numbers that represent the amplitude of thephysical quantities. In some embodiments, ADC 1402 converts the analogoutput of step down converter 1401 to its corresponding digitalrepresentation. Any suitable ADC may be used to implement ADC 1402. Forexample, ADC 1402 is one of: direct-conversion ADC (for flash ADC),two-step flash ADC, successive-approximation ADC (SAR ADC), ramp-compareADC, Wilkinson ADC, integrating ADC, delta-encoded ADC or counter-ramp,pipeline ADC (also called subranging quantizer), sigma-delta ADC (alsoknown as a delta-sigma ADC), time-interleaved ADC, ADC with intermediateFM stage, or time-stretch ADC. For purposes of explaining the variousembodiments, ADC 1402 is considered to be flash ADC.

In some embodiments, PLL 1403 is one of: an analog PLL, digital PLL,mixed signal PLL (e.g., having analog and digital components),Inductor-capacitor tank (LC) PLL, self-biased PLL, etc.

FIGS. 15A-B illustrate flowcharts of a method operation of the waterheater, according to some embodiments of the disclosure. It is pointedout that those elements of FIGS. 15A-B having the same reference numbers(or names) as the elements of any other figure can operate or functionin any manner similar to that described, but are not limited to such.

Although the blocks in the flowchart with reference to FIGS. 15A-B areshown in a particular order, the order of the actions can be modified.Thus, the illustrated embodiments can be performed in a different order,and some actions/blocks may be performed in parallel. Some of the blocksand/or operations listed in FIGS. 15A-B are optional in accordance withcertain embodiments. The numbering of the blocks presented is for thesake of clarity and is not intended to prescribe an order of operationsin which the various blocks must occur. Additionally, operations fromthe various flows may be utilized in a variety of combinations.

The process begins to at block 1501. At block 1501, a determination ismade whether upper relay direction flag is positive or negative. In someembodiments, the upper relay direction flag is used to model theoperation of the upper standard water heater relay. The relay has ahysteresis characteristic. For example, if the relay is on, it turns offat a different point that it would turn on if it were off as shown by1210 of FIG. 12. The upper relay direction flag tracks whether the relayis at curve 1210 a or curve 1210 b of plot 1210.

Referring back to FIGS. 15A-B, if upper relay direction flag ispositive, then the process proceeds to block 1502. At block 1502, adetermination is made whether the upper tank temperature (temp) isgreater than upper tank upper temperature limit. If the upper tanktemperature is greater than the upper tank upper temperature limit, theprocess proceeds to block 1507. At block 1507, zero power (i.e., nopower) is applied to upper element 505 and upper relay direction flag isset negative. The process then proceeds to block 1508. If upper relaydirection flag is negative, then the process proceeds to block 1503.

At block 1503, a determination is made whether the upper tanktemperature is less than the upper tank lower temperature limit. If theupper tank temperature is less than the upper tank lower temperaturelimit, the process proceeds to block 1504. At block 1504, power isapplied to upper element 505 (while zero power or no power is applied tolower element 507). The upper relay direction flag is then set positive.The process them proceeds to block 1501.

If the upper tank temperature is greater than or equal to the upper tanklower temperature limit, the process proceeds to block 1506. At block1506, zero power (or no power) is applied to upper element 505. Theprocess them proceeds to block 1508. At block 1508, a determination ismade whether the lower tank temperature is greater than the lower tanktemperature limit. If the lower tank temperature is greater than thelower tank temperature limit, the process proceeds to block 1509. Atblock 1509, zero power (or no power) is applied lower element 507. Theprocess then proceeds to block 1501. If the upper tank temperature isless than or equal to the upper tank upper temperature limit, theprocess proceeds to block 1505. At block 1505, power is applied to theupper element 505 while zero power is applied to lower element 507. Theprocess then proceeds to block 1501.

If the lower tank temperature is less than or equal to the lower tanktemperature limit, the process proceeds to block 1510 as indicated byreference sign ‘A’. At block 1510, parameter ‘A’ is calculated as K₁*(lower temperature set point—lower tank temperature), where K₁ is thegain of block 1405 of FIG. 14. Referring back to FIG. 15B, the processthen proceeds to block 1511. At block 1511, parameter ‘B’ is calculatedas K₂*(grid frequency-grid frequency setpoint), where K₂ is the gain ofblock 1408 of FIG. 14. Referring back to FIG. 15B, the process thenproceeds to block 1512. At block 1512, a determination is made whether asum of parameters ‘A’ and ‘B’ (i.e., A+B) is greater than maximum lowerelement power rating. If the sum of parameters ‘A’ and ‘B’ is greaterthan maximum lower element power rating, the process proceeds to block1513.

At block 1513, maximum rated power is applied to lower element 507. Theprocess then proceeds to block 1501 as indicated by reference sign ‘B’.If the sum of parameters ‘A’ and ‘B’ is less than or equal to themaximum lower element power rating, the process proceeds to block 1514.At block 1514, a determination is made whether the sum of parameters of‘A’ and ‘B’ is less than zero. If the sum of parameters ‘A’ and ‘B’ isless than zero, the process proceeds to block 1515. At block 1515, zerorated power is applied to lower element 507. The process then proceedsto block 1501 as indicated by reference sign ‘B’. If the sum ofparameters ‘A’ and ‘B’ is greater than or equal to zero, the processproceeds to block 1516. At block 1516, power proportional (or equal) tothe sum of ‘A’ and ‘B’ is applied to lower element 507. The process thenproceeds to block 1501 as indicated by reference sign ‘B’.

Elements of embodiments are also provided as a machine-readable mediumfor storing the computer-executable instructions. The machine-readablemedium may include, but is not limited to, flash memory, optical disks,CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards,phase change memory (PCM), or other types of machine-readable mediasuitable for storing electronic or computer-executable instructions. Forexample, embodiments of the disclosure may be downloaded as a computerprogram (e.g., BIOS) which may be transferred from a remote computer(e.g., a server) to a requesting computer (e.g., a client) by way ofdata signals via a communication link (e.g., a modem or networkconnection).

Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” or “other embodiments” means that a particularfeature, structure, or characteristic described in connection with theembodiments is included in at least some embodiments, but notnecessarily all embodiments. The various appearances of “an embodiment,”“one embodiment,” or “some embodiments” are not necessarily allreferring to the same embodiments. If the specification states acomponent, feature, structure, or characteristic “may,” “might,” or“could” be included, that particular component, feature, structure, orcharacteristic is not required to be included. If the specification orclaim refers to “a” or “an” element, that does not mean there is onlyone of the elements. If the specification or claims refer to “anadditional” element, that does not preclude there being more than one ofthe additional element.

Furthermore, the particular features, structures, functions, orcharacteristics may be combined in any suitable manner in one or moreembodiments. For example, a first embodiment may be combined with asecond embodiment anywhere the particular features, structures,functions, or characteristics associated with the two embodiments arenot mutually exclusive.

While the disclosure has been described in conjunction with specificembodiments thereof, many alternatives, modifications and variations ofsuch embodiments will be apparent to those of ordinary skill in the artin light of the foregoing description. The embodiments of the disclosureare intended to embrace all such alternatives, modifications, andvariations as to fall within the broad scope of the appended claims.

In addition, well known power/ground connections to integrated circuit(IC) chips and other components may or may not be shown within thepresented figures, for simplicity of illustration and discussion, and soas not to obscure the disclosure. Further, arrangements may be shown inblock diagram form in order to avoid obscuring the disclosure, and alsoin view of the fact that specifics with respect to implementation ofsuch block diagram arrangements are highly dependent upon the platformwithin which the present disclosure is to be implemented (i.e., suchspecifics should be well within purview of one skilled in the art).Where specific details (e.g., circuits) are set forth in order todescribe example embodiments of the disclosure, it should be apparent toone skilled in the art that the disclosure can be practiced without, orwith variation of, these specific details. The description is thus to beregarded as illustrative instead of limiting.

The following examples pertain to further embodiments. Specifics in theexamples may be used anywhere in one or more embodiments. All optionalfeatures of the apparatus described herein may also be implemented withrespect to a method or process.

For example, an energy storing apparatus is provided which comprises: afirst region having a thermostat which includes: a temperature sensor; acontroller coupled to the temperature sensor; and a power-convertercoupled to the controller; and a second region thermally coupled to thefirst region. In some embodiments, the power-converter is to control theamount of power applied to the first region. In some embodiments, thepower-converter is operable to apply power to the first region accordingto temperature sensed by the temperature sensor and a frequency of anelectric grid. In some embodiments, the energy storing apparatuscomprises: a first proportional loop to regulate a temperature sensed bythe temperature sensor; and a second proportional loop to adjustfrequency of an electric grid. In some embodiments, the first and secondproportional loops to operate in parallel. In some embodiments, thesecond proportional loop is to emulate inertia of the electric grid. Insome embodiments, the first region has set point which is set to atemperature lower than a temperature of the second region,

In another example, a method for controlling frequency of an electricgrid is provided. In some embodiments, the method comprises: determiningwhether an upper relay direction flag is positive or negative, and ifnegative, determining whether an upper tank temperature is less than anupper tank lower temperature limit, and if positive, determining whetheran upper tank temperature is greater than an upper tank uppertemperature limit; and applying power to an upper heating element,setting the upper relay direction flag to positive, and applying zeropower to a lower heating element in response to determining that theupper tank temperature is less than the upper tank lower temperaturelimit, wherein the upper and lower heating elements are part of a waterheater. In some embodiments, the method comprises: applying zero powerto the upper heating element in response to determining that the uppertank temperature is greater than or equal to the upper tank lowertemperature limit.

In some embodiments, the method comprises: applying zero power to theupper heating element and setting upper relay direction flag to negativein response to determining that the upper tank temperature is greaterthan the upper tank upper temperature limit; or applying power to theupper heating element and applying no power to the lower heating elementin response to determining that the upper tank temperature is less thanor equal to the upper tank upper temperature limit. In some embodiments,the method comprises determining whether a lower tank temperature isgreater than a lower tank temperature limit. In some embodiments, themethod comprises: applying zero power to the lower heating element inresponse to determining that the lower tank temperature is greater thanthe lower tank temperature limit; or computing first and secondparameters in response to determining that the lower tank temperature isless than or equal to the lower tank temperature limit, wherein thefirst parameter is a function of the lower tank temperature and a lowertemperature set point, and wherein the second parameter is a function ofthe frequency of the electric grid and a frequency set point of theelectric grid.

In some embodiments, the method comprises: determining whether a sum ofthe first and second parameters is greater than a maximum lower heatingelement power rating; and applying a maximum rated power to the lowerheating element in response to determining that the sum of the first andsecond parameters is greater than the maximum lower heating elementpower rating. In some embodiments, the method comprises: determiningwhether a sum of the first and second parameters is less than zero;applying zero rated power to the lower heating element in response todetermining that the sum of the first and second parameters is less thanzero; or applying power to the lower element in response to determiningthat the sum of the first and second parameters is greater than or equalto zero, wherein the applied power is a function of the sum of the firstand second parameters.

In another example, an apparatus is provided which comprises: aninterface to be coupled to an electric grid; a temperature sensor; acontroller coupled to the temperature sensor; and a power-convertercoupled to the controller, wherein the power-converter is to couple viapassive devices to a heating element, wherein the power-converter is toapply power to the heating element according to a temperature sensed bythe temperature sensor and a frequency of the electric grid. In someembodiments, the apparatus comprises at least two diodes coupled to theinterface. In some embodiments, the power-converter includes at leasttwo transistors coupled in series such than a common node of the leasttwo transistors is coupled to at least one of the passive devices, andwherein the power-converter is to control an amount of power applied tothe heating element.

In some embodiments, the apparatus comprises: a first proportional loopto regulate a temperature sensed by the temperature sensor; and a secondproportional loop to adjust frequency of an electric grid. In someembodiments, the first and second proportional loops are to operate inparallel. In some embodiments, the second proportional loop is toemulate inertia of the electric grid.

An abstract is provided that will allow the reader to ascertain thenature and gist of the technical disclosure. The abstract is submittedwith the understanding that it will not be used to limit the scope ormeaning of the claims. The following claims are hereby incorporated intothe detailed description, with each claim standing on its own as aseparate embodiment.

What is claimed is:
 1. An energy storing apparatus comprising: a firstregion having a thermostat which includes: a temperature sensor; acontroller coupled to the temperature sensor; and a power-convertercoupled to the controller; and a second region thermally coupled to thefirst region.
 2. The energy storing apparatus of claim 1, wherein thepower-converter is to control the amount of power applied to the firstregion.
 3. The energy storing apparatus of claim 2, wherein thepower-converter is operable to apply power to the first region accordingto temperature sensed by the temperature sensor and a frequency of anelectric grid.
 4. The energy storing apparatus of claim 1 comprises: afirst proportional loop to regulate a temperature sensed by thetemperature sensor; and a second proportional loop to adjust frequencyof an electric grid.
 5. The energy storing apparatus of claim 4, whereinthe first and second proportional loops are to operate in parallel. 6.The energy storing apparatus of claim 4, wherein the second proportionalloop is to emulate inertia of the electric grid.
 7. The energy storingapparatus of claim 1, wherein the first region has a set point which isset to a temperature lower than a temperature of the second region.
 8. Amethod for controlling frequency of an electric grid, the methodcomprising: determining whether an upper relay direction flag ispositive or negative, and if negative, determining whether an upper tanktemperature is less than an upper tank lower temperature limit, and ifpositive, determining whether an upper tank temperature is greater thanan upper tank upper temperature limit; and applying power to an upperheating element, setting the upper relay direction flag to positive, andapplying zero power to a lower heating element in response todetermining that the upper tank temperature is less than the upper tanklower temperature limit, wherein the upper and lower heating elementsare part of a water heater.
 9. The method of claim 8 comprising:applying zero power to the upper heating element in response todetermining that the upper tank temperature is greater than or equal tothe upper tank lower temperature limit.
 10. The method of claim 9comprising: applying zero power to the upper heating element and settingthe upper relay direction flag to negative in response to determiningthat the upper tank temperature is greater than the upper tank uppertemperature limit; or applying power to the upper heating element andapplying no power to the lower heating element in response todetermining that the upper tank temperature is less than or equal to theupper tank upper temperature limit.
 11. The method of claim 10comprising: determining whether a lower tank temperature is greater thana lower tank temperature limit.
 12. The method of claim 11 comprising:applying zero power to the lower heating element in response todetermining that the lower tank temperature is greater than the lowertank temperature limit; or computing first and second parameters inresponse to determining that the lower tank temperature is less than orequal to the lower tank temperature limit, wherein the first parameteris a function of the lower tank temperature and a lower temperature setpoint, and wherein the second parameter is a function of the frequencyof the electric grid and a frequency set point of the electric grid. 13.The method of claim 12 comprising: determining whether a sum of thefirst and second parameters is greater than a maximum lower heatingelement power rating; and applying a maximum rated power to the lowerheating element in response to determining that the sum of the first andsecond parameters is greater than the maximum lower heating elementpower rating.
 14. The method of claim 13 comprising: determining whethera sum of the first and second parameters is less than zero; applyingzero rated power to the lower heating element in response to determiningthat the sum of the first and second parameters is less than zero; orapplying power to the lower heating element in response to determiningthat the sum of the first and second parameters is greater than or equalto zero, wherein the applied power is a function of the sum of the firstand second parameters.
 15. An apparatus comprising: an interface to becoupled to an electric grid; a temperature sensor; a controller coupledto the temperature sensor; and a power-converter coupled to thecontroller, wherein the power-converter is to couple via passive devicesto a heating element, wherein the power-converter is to apply power tothe heating element according to a temperature sensed by the temperaturesensor and a frequency of the electric grid.
 16. The apparatus of claim15 comprises at least two diodes coupled to the interface.
 17. Theapparatus of claim 15, wherein the power-converter includes at least twotransistors coupled in series such than a common node of the least twotransistors is coupled to at least one of the passive devices, andwherein the power-converter is to control an amount of power applied tothe heating element.
 18. The apparatus of claim 15 comprises: a firstproportional loop to regulate a temperature sensed by the temperaturesensor; and a second proportional loop to adjust frequency of anelectric grid.
 19. The apparatus of claim 18, wherein the first andsecond proportional loops are to operate in parallel.
 20. The apparatusof claim 18, wherein the second proportional loop is to emulate inertiaof the electric grid.